The art is aware that a number of commercially useful compositions are used to produce various workpieces by inducing a transformation in the composition at some point in the production of workpiece for use. In many cases, such a transformation involves subjecting the composition/workpiece to conditions that can cause processing-related defects in the composition and/or the workpiece produced. If the manufacturing process is not properly controlled, such processing-related defects can prevent the workpiece from being economically or effectively produced and/or can introduce defects into the workpiece. This is especially true in the field of chemical compositions, where the transformation steps often involve removal of significant amounts of solvent and/or changes that induce conformational changes in the composition during the transformation. Poly(amic acid) and polyimide compositions are exemplary of this phenomenon. The workpiece can take on a variety of forms, such as a film, membrane or three-dimensional shape.
Polyimides are an important class of polymeric materials and are known for their superior performance characteristics. Most polyimides are comprised of relatively rigid molecular structures with aromatic/cyclic moieties and exhibit high glass transition temperatures, good mechanical strength, high Young's modulus, and excellent thermo-oxidative stability. Furthermore, the linearity and stiffness of the cyclic/aromatic backbone reduce segmental rotation and allow for molecular ordering which results in lower coefficients of thermal expansion (CTE) than those thermoplastic polymers having more flexible chains. In addition, the intermolecular associations of polyimide chains provide resistance to most solvents.
As a result of their favorable characteristics, polyimide compositions have become widely used in the aerospace industry, the electronics industry and the telecommunications industry. In the electronics industry, polyimide compositions are used in applications such as forming protective and stress buffer coatings for semiconductors, dielectric layers for multilayer integrated circuits and multi-chip modules, high temperature solder masks, bonding layers for multilayer circuits, final passivating coatings on electronic devices, and the like. In addition, polyimide compositions may form dielectric films in electrical and electronic devices such as motors, capacitors, semiconductors, printed circuit boards and other packaging structures. Polyimide compositions may also serve as an interlayer dielectric in both semiconductors and thin film multichip modules. The low dielectric constant, low stress, high modulus, and inherent ductility of polyimide compositions make them well suited for these multiple layer applications. Other uses for polyimide compositions include alignment and/or dielectric layers for displays, and as a structural layer in micromachining applications.
Furthermore, in the aerospace industry, polyimide compositions are used for optical applications as membrane reflectors and the like. In application, a polyimide composition is secured by a metal (often aluminum, copper, or stainless steel) or composite (often graphite/epoxy or fiberglass) mounting ring that secures the border of the polyimide compositions. Such optical applications may be used in space, where the polyimide compositions and the mounting ring are subject to repeated and drastic heating and cooling cycles in orbit as the structure is exposed to alternating periods of sunlight and shade.
Polyimide compositions may be synthesized by a number of methods that are known in the art. Exemplary of such methods is the traditional two-step method of synthesizing polyimide compositions, in which a solution of the aromatic diamine in a polar solvent, such as, but not limited to, N-methylpyrrolidone (NMP), is prepared. To this solution a tetracarboxylic acid, usually in the form of a dianhydride, is added. The diamine and the tetracarboxylic acid are generally added in a 1:1 molar stoichiometry, although other stoichiometries may be used. The resulting polycondensation reaction forms a poly(amic acid). The high molecular weight poly(amic acid) acid produced is soluble in the reaction solvent and, therefore, the solution may be cast into a film on a suitable substrate, such as by spin casting, or processed in other ways to produce the final polyimide composition. One common form of polyimide compositions is a polyimide film or membrane. The polyimide film may be produced by casting the soluble poly(amic acid) produced onto a substrate. The cast film is then further processed to remove the solvent and/or to convert the amic acid functional groups to imides with a cyclodehydration reaction, also called imidization.
Several methods are known in the prior art for accomplishing the imidization reaction. In one method, the cast film is heated, generally in stages, to elevated temperatures to remove solvent and accomplish imidization. Alternatively, some poly(amic acids) may be converted in solution to soluble polyimides by using a chemical dehydrating agent, catalyst, and/or heat. Other methods may also be used in certain cases.
During the imidization process, the amic acid functional groups on the poly(amic acid) undergo a chemical conversion to the imide groups in the polyimide. This conversion can impact the nature of the polyimide compositions. Poly(amic acid) compositions have more conformational freedom than their corresponding polyimide compositions. Therefore, during imidization, the conformational freedom of the constituents of the polyimide compositions is reduced. In addition, significant quantities of solvent may also be removed which can cause a reduction in conformational freedom and reduction in volume of the resulting compositions. In certain cases, soluble polyimide compositions may also be used to prepare a polyimide workpiece, such as a polyimide film. The polyimide film may be produced by casting the soluble polyimide onto a substrate. The cast film is then further processed to remove the solvent. During processing, the solvent removal causes a significant stress to accumulate as discussed above.
These changes, either alone or in combination with each other and/or other factors, causes a significant stress to accumulate in the produced polyimide compositions. The amount of accumulated stress is dependent in part on the chemical characteristics of the poly(amic acid) and the resulting polyimide compositions, on the amount of solvent present in the poly(amic acid) solution, the amount of solvent removed during processing and on the physical characteristics of the resulting polyimide compositions, such as but not limited to, size and thickness. For rigid, low CTE polyimide compositions, the difference in conformational freedom is quite significant. As a general rule, the lower the CTE of the polyimide compositions within a copolymer family, the more stress buildup occurs during further processing.
In traditional methods of casting polymer compositions (including films), such as but not limited to polyimide compositions, the poly(amic acid) solution is in contact with the substrate on which the poly(amic acid) solution is cast and is not capable of sufficient movement to release the accumulated stress as the imidization and/or solvent removal processes occurs. As such, as the poly(amic acid) compositions undergo the conformational changes as a result of imidization and/or solvent removal, the forming polyimide compositions are unable to release such accumulated stress without damaging the final product (i.e., the workpiece). The release of accumulated stress is a particular problem with polymer compositions, such as but not limited to, polyimide compositions, having a thickness of about 0.8 mils or greater.
A number of methods have been used to address the issue of substrate adherence in relation to polyimide compositions. For example, release interface agents have been used. The release interface agents are applied to the substrate prior to the addition of the polyimide or poly(amic acid) solution. The release interface agents thereby form a layer between the polyimide or poly(amic acid) solution and the substrate. While effective at providing enhanced release from the substrate, in many cases the release interface agent transfers to the final workpiece producing a workpiece that is hazy or cloudy (such modifications may result in the workpiece being unsuitable for use). Additionally, in many cases, use of the release interface agents results in polyimide films that self-release from the substrate during cure due to the stress accumulation. In many cases, this self-release phenomenon is not desirable and results in the deformation of the workpiece.
The accumulated stress may result in processing-related defects in the final polymer workpiece as discussed. Such processing-related defects include, but are not limited to, cracking, tearing, curling, warping, and mechanical anisotropy (having properties that differ according to the direction of measurement). Such processing-related defects can render the polymer workpiece unsuitable for the use for which it was originally intended.
While the discussion and examples of the present disclosure center on poly(amic acid) and/or polyimide compositions and the problems associated with manufacturing poly(amic acid) and/or polyimide workpieces, similar problems are known in the manufacture of other compositions, in particular, chemical compositions and polymer compositions, and the teachings of the present disclosure should not be limited to polyimide compositions.
Therefore, the art is lacking a method for the preparation of compositions and/or workpieces, such as, but not limited to, poly(amic acid)/polyimide compositions and/or workpieces, that addresses the problems noted in the art. The present disclosure provides a novel method useful in preparing a variety of compositions and/or workpieces, such as, but not limited to, poly(amic acid)/polyimide compositions and/or workpieces, that addresses the problems of processing-related defects, such as, but not limited to, in the case of poly(amic acid)/polyimide compositions and/or workpieces, transformation-related defects and substrate retention-related defects. As a result, the use of the methods of the present disclosure provides compositions and/or workpieces that show reduced processing-related defects, are more consistent in chemical and physical properties and are cheaper and more economical to produce (since less product is rejected as unsuitable). Furthermore, the use of the methods of the present disclosure allows a wider variety of compositions and/or workpieces to be produced using a wider variety of starting materials. With reference to poly(amic acid)/polyimide compositions and/or workpieces, the present disclosure allows a wider variety of polyimide and poly(amic acid) solutions to be utilized and a corresponding wider variety of polyimide compositions to be produced.